The need to reduce air pollution hazards caused by volatile organic solvents has increased the importance of water-borne resin coating systems.
As described in U.S. Pat. Nos. 4,048,179 and 4,073,762, various resins have been developed for aqueous paint compositions, and epoxy resins are often employed when such coating properties as rust prevention and chemical resistance are regarded as essential for particular applications.
U.S. Pat. No. 3,238,170 describes an aqueous epoxy resin composition comprising (1) a polyepoxy resin and (2) a copolymer of an alkyl ester of an .alpha.,.beta.-ethylenically unsaturated monocarboxylic acid and an alkyl half ester of itaconic acid, which is an aqueous coating composition having good alkali resistance, adhesion and storage stability.
An aqueous emulsion comprising an epoxy resin and a protein acting as protective colloid, and an emulsion comprising a mixture of an amine-epoxy resin condensate and an epoxy resin, are disclosed in U.S. Pat. Nos. 3,020,250 and 3,449,281.
Further, attention has been directed to various emulsifiers which are adapted for emulsification of epoxy resins in water. For example, an aqueous emulsion of an epoxy resin in which a reaction product of a boric acid ester derived from boric acid with both an alkylene glycol and a .beta.-dialkyl-substituted aminoalkanol is employed as an emulsifier is described in U.S. Pat. No. 3,301,804. A polyepoxide emulsion for electrodeposition in which a polyepoxide is emulsified with an emulsifying agent of the phosphate ester type is described in U.S. Pat. No. 3,634,348. An epoxy resin emulsion for sizing glass fibers in which a cationic emulsifying agent selected from the group consisting of imidazolines and amides and a non-ionic emulsifying agent are used in combination in U.S. Pat. No. 3,249,412.
Also, various curing agents for epoxy resin emulsion compositions are known in the art. For example, curing agents for epoxy resin compositions include a polyamide reaction product derived from a polymeric fatty acid and an aliphatic polyamine containing terminal amino groups (U.S. Pat. No. 2,811,495 and U.S. Pat. No. 2,899,397), a salted amine derived from a carboxylic acid having 1-8 carbon atoms and a tertiary amine (U.S. Pat. No. 3,640,926), a polyamide derived from a diamine and a dicarboxylic acid (U.S. Pat. No. 3,355,409), a phenol modified polyamine (U.S. Pat. No. 3,383,347), an amino-containing polyamide prepared by reacting a polyamine with a polymerized fatty acid (U.S. Pat. No. 3,324,041), and the like.
In most cases a bisphenol type of epoxy resin is employed as a film-forming component of an aqueous epoxy resin paint composition. However, a bisphenol type of epoxy resin as an aqueous dispersion component normally has the disadvantage that it tends to crystallize and settle out when the aqueous dispersion is stored for three months or longer under ambient conditions.
Destruction of emulsion particles usually occurs with advance of crystallization, and thereafter it is impossible to restore the original state even by heating. Thus, an aqueous epoxy resin paint has the disadvantage that if crystallization occurs the crystallized paint cannot be used for coating purposes.
Some recent developments relate to the production of water-borne epoxy resin systems which exhibit improved properties under ambient conditions.
U.s. Pat. No. 4,048,179 describes a process for preparing water-dilutable, heat-curing coating compositions comprising (a) reacting an aromatic polyglycidyl ether with a polyethylene glycol in the presence of 0 to 20% of a polyhydric phenol, (b) adding an aminoplast curing agent, and (c) diluting with water to give a stable aqueous composition.
U.S. Pat. No. 4,073,762 describes an aqueous epoxy resin paint composition comprising an epoxy resin emulsion formed by emulsifying in water (a) 98 to 50% by weight of a bisphenol type epoxy resin and (b) 2 to 50% by weight novolak type epoxy resins, with a non-ionic surface active agent, and a curing agent incorporated in said epoxy resin emulsion.
There remains a need for improved stable aqueous epoxy resin dispersions which are adapted for application as industrial maintenance coating systems.
Accordingly, it is an object of this invention to provide an aqueous dispersion of a bisphenol A type epoxy resin, which exhibits long term stability under ambient storage conditions.
It is another object of this invention to provide a freeze-thaw resistant water-borne paint composition containing self-emulsifying epoxy resin, which paint composition can be applied as a coating that cures at room temperature to form a continuous thermoset film.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.